BLDC (Brushless DC) motors are replacing DC motors and induction motors due to its efficiency and reliability. Unlike DC motors, BLDC motors need commutation based on rotor position. Hall effect sensors have been used to detect rotor position. However, mounting Hall effect sensors and processing signals from those sensors require additional cost. Moreover, a Hall effect sensor and its wiring can reduce the reliability of a motor drive system. Due to these additional costs and reliability concerns there has been interest in developing BLDC motors without position sensors.
A common way of estimating rotor position without position sensors is by measuring back electromotive force (BEMF) voltage. BEMF voltage of a motor varies according to rotor position. Measuring BEMF voltage also requires the use of a trapezoidal pulse width modulation (PWM) drive to run the motor. With a trapezoidal drive, BEMF voltage can be measured on the floating phase terminal during a no-switching period. Based on this measurement, a zero crossing point of the BEMF voltage can be detected. The zero voltage point can be used to predict a time required for a next commutation cycle.
The terminal voltage measured during the no-switching period is not the motor phase voltage. The neutral point voltage should be taken into consideration to detect correctly the zero crossing point of the BEFM voltage. Typically, the neutral point of a motor's three-phase winding is not exposed in most applications. Conventionally, three resistors have been used to simulate the neutral point. In addition to these resistors, resistor-capacitor (RC) filters are used to filter out PWM noise components from the sensed BEMF voltage. This filtering can result in sensing delay, which has a negative influence on high-speed motor performance. In addition, the additional filter components can affect cost, reliability and consume valuable printed circuit board (PCB) area.
Some conventional methods propose using simpler hardware by measuring phase voltage when the neutral voltage goes to low. These methods, however, need some amount of turn-off period, which prevents operation of the motor at high speed. Other conventional methods propose measuring phase current information. These methods, however, require additional hardware and CPU resources and are not suitable for applications where cost is a critical factor for product success.